1. Field of the Invention
The present invention relates to an improvement on a wafer holding jig for vacuum-holding a semiconductor wafer while the wafer is ground or polished.
2. Description of the Related Art
Conventionally, when a wafer such as a silicon wafer or a GaAs wafer is ground or polished, a wafer holding jig formed from a fine-grain sintered body-which has a high strength and which therefore does not deform due to machining pressure-is used to accurately machine the surface of the wafer into a highly flat surface.
Also, there has been known a holding jig having a plurality of regions having different characters. As shown in FIG. 5, in such a holding jig 51, a wafer holding surface for holding a wafer W is formed by a porous fine-grain sintered body 52 and a nonporous fine-grain sintered body 53 surrounding the porous fine-grain sintered body 52. The porous fine-grain sintered body 52 is formed by a process in which fine grains are sintered such that a resultant sintered body becomes porous. The nonporous fine-grain sintered body 53 is formed by a process in which fine grains are sintered such that a resultant sintered body becomes dense or nonporous. Evacuation passages 54 are formed to communicate with the porous fine-grain sintered body 52. The wafer W is vacuum-held by means of evacuation through the evacuation passages 54. In such a holding jig, in order to define a vacuum region, the outer diameter of the porous fine-grain sintered body 52 is generally made smaller than the diameter of the wafer W, so that the vacuum region is formed by the outside nonporous fine-grain sintered body 53 and the wafer W.
In general, even when the holding surface of a holding jig is flat, a wafer held by the holding jig deforms if foreign matter such as dust is caught between the wafer and the holding surface of the holding jig. This becomes a cause of a deterioration in machining accuracy.
In the case of the above-described vacuum type wafer holding jig, since dust caught between the porous fine-grain sintered body 52 and the wafer W is sucked through pores in the surface of the porous fine-grain sintered body 52, no problems occur. However, since dust caught between the wafer W and the nonporous fine-grain sintered body 53, which holds the outer circumferential portion of the wafer is not sucked, the wafer W may be held in a deformed state.
When the wafer is machined in such a state, the flatness of the surface of the wafer deteriorates.
Further, the amount of deformation of the wafer W due to application of machining pressure varies between the portion held by the porous fine-grain sintered body 52 and the portion held by the nonporous fine-grain sintered body 53. Since the amount of load-induced deformation at the portion held by the fine-grain porous sintered body 52 is greater than that at the portion held by the nonporous fine-grain sintered body 53, the stock removal of the machining (amount of material removed by machining) at the center portion becomes smaller. Therefore, the machined wafer has a problem of insufficient flatness (flatness defect) in which the center portion has a lager thickness than does the remaining portion of the wafer.
Therefore, there has been a strong desire for a technique for preventing a deterioration in machining accuracy, which deterioration would otherwise occur due to deformation of the wafer stemming from catch of dust or the like, or unevenness in deformation generated upon application of machining pressure to the wafer.